The present invention relates to bipolar transistors, and more particularly to a silicon germanium (SiGe) bipolar transistor which includes a lightly doped Si collector region and a SiGe base region that include carbon, C, continuously incorporated throughout the collector and SiGe base regions. A method of continuously incorporating C into the lightly doped Si collector region and SiGe base region of a SiGe bipolar transistor is also disclosed herein. The term SiGe is used herein to denote silicon-germanium alloys, i.e., Si1-xGex.
Significant growth in both high-frequency wired and wireless markets has introduced new opportunities where compound semiconductors such as SiGe have unique advantages over bulk complementary metal oxide semiconductor (CMOS) technology. With the rapid advancement of epitaxial-layer pseudomorphic SiGe deposition processes, epitaxial-base SiGe heterojunction bipolar transistors have been integrated with mainstream advanced CMOS development for wide market acceptance, providing the advantages of SiGe technology for analog and RF circuitry while maintaining the full utilization of the advanced CMOS technology base for digital logic circuitry.
SiGe heterojunction bipolar transistor devices are replacing silicon bipolar junction devices as the primary element in all analog applications. A typical prior art SiGe heterojunction bipolar transistor is shown in FIG. 1. Specifically, the prior art heterojunction bipolar transistor includes an n+ subcollector layer 10 having a layer of nxe2x88x92 Si collector (i.e., lightly doped) region 12 formed thereon. The transistor further includes p+ SiGe base region 14 formed on the lightly doped Si collector region. One portion of base region 14 includes n+ Si emitter region 16 and other portions include base electrodes 18 which are separated from the emitter region by spaces 20. On top of emitter region 16 is an emitter electrode 22.
A major problem with bipolar SiGe transistors of the type illustrated in FIG. 1 is the presence of dislocations between the collector and emitter regions. When these dislocations extend between the collector region and the emitter region, bipolar pipe, e.g., CE, shorts occur; Pipe shorts are a major yield detractor in SiGe bipolar technology.
In the prior art, it is known to incorporate carbon into a bipolar structure so as to form a carbon layer over the base in the SiGe region only. Such a structure is shown in FIG. 2 wherein reference numeral 24 denotes the grown carbon layer. This prior art technique which forms a C layer over the base in the SiGe region results in a narrow base width by hindering diffusion of the intrinsic base region. This result is shown, for example, in FIG. 3.
Carbon incorporation is typically employed in the prior art to prevent the out-diffusion of boron into the base region. For example, it is known that the transient enhanced diffusion of boron is strongly suppressed in a carbon-rich silicon layer, See H. J. Osten, et al., xe2x80x9cCarbon Doped SiGe Heterjunction Bipolar Transistors for High Frequency Applicationsxe2x80x9d, IEEEBTCM 7.1, 109. Boron diffusion in silicon occurs via an interstitial mechanism and is proportional to the concentration of silicon self-interstitials. Diffusion of carbon out of the carbon-rich regions causes an undersaturation of silicon self-interstitials. As a result, the diffusion of boron in these regions is suppressed. Despite being able to suppress the diffusion of boron, this prior art method which forms C over the base in the SiGe region only is not effective in reducing pipe shorts.
In view of the bipolar pipe shorts problem mentioned above, there is a continued need for the development of a new and improved method for fabricating SiGe bipolar transistors in which dislocations between the emitter and collector regions are substantially eliminated, without narrowing the base width as is the case with prior art methods.
One object of the present invention is to provide a method of fabricating a SiGe bipolar transistor in which the formation of dislocations between the emitter and collector regions have been substantially suppressed therefore avoiding the problem of bipolar pipe, e.g., CE, shorts.
Another object of the present invention is to provide a method of fabricating a SiGe bipolar structure in which the transistor yield of epitaxially grown silicon/SiGe region is enhanced.
A further object of the present invention is to provide a method of fabricating a bipolar SiGe transistor in which carbon can be incorporated into the structure without narrowing the base width.
A yet further object of the present invention is to provide a method of fabricating a bipolar SiGe transistor which is cost effective and that can be easily implemented with existing SiGe bipolar technology.
These and other objects and advantages are achieved in the present invention by incorporating carbon into the lightly doped Si layer as well as the SiGe base region. In accordance with the present invention, the C incorporation occurs during the epitaxial growth of the SiGe layer by using a deposition process such as ultra-high vacuum chemical vapor deposition (UHVCVD), rapid thermal chemical vapor deposition (RTCVD), molecular beam epitaxy (MBE), or plasma-enhanced chemical vapor deposition (PECVD),wherein a carbon source gas is employed. By employing the inventive method, carbon is continuously formed throughout the Si collector region and the SiGe base region. Moreover, applicants have found that the inventive method provides enhanced yield of SiGe as well as suppressing dislocations which cause bipolar pipe shorts.
In one aspect of the present invention, a method of fabricating a SiGe bipolar transistor which exhibits essentially no pipe shorts is provided. Specifically, the inventive method of fabricating the SiGe bipolar transistor includes the steps of:
(a) providing a structure which includes at least a bipolar device region, said bipolar device region including at least a collector region of a first conductivity type formed in a semiconductor substrate;
(b) depositing a SiGe base region on said collector region, wherein during said depositing carbon is continuously grown through the collector region and the SiGe base region; and
(c) forming a patterned emitter region over said SiGe base region.
In accordance with the above described method, depositing step (b) may include UHVCVD, MBE, RTCVD, PECVD or another like deposition process which is capable of forming a SiGe base region. Of these deposition processes, it is preferred to use a UHVCVD process.
Another aspect of the present invention relates to a method of incorporating C into the collector region and the SiGe base region of a bipolar transistor. In accordance with this aspect of the present invention, the method includes a step of depositing a SiGe base region on a lightly doped Si collector region, wherein during said depositing carbon is continuously grown through the collector region and the SiGe base region.
In accordance with the this aspect of the present invention, the SiGe base region may be formed by UHVCVD, MBE, RTCVD, PECVD or another like deposition process which is capable of forming such a region. Of these deposition processes, it is preferred to use a UHVCVD process.
A further aspect of the present invention relates to a SiGe bipolar transistor which includes substantially no dislocation defects present between the emitter and collector region, said structure comprising:
a collector region of a first conductivity type;
a SiGe base region; and
an emitter region of said first conductivity type formed over a portion of said base region, wherein said collector region and said base region include carbon continuously present in said collector and SiGe base regions and said SiGe base is further doped with B.